Solid-state semiconductor lasers represent desirable light sources for a variety of applications including optical data communications, telecommunications, and other applications. A vertical cavity surface emitting laser (VCSEL) is a solid-state semiconductor laser in which light is emitted from the surface of a monolithic structure of semiconductor layers, in a direction normal to the surface. This is in contrast to the more commonly used edge-emitting laser, in which light is emitted from the edge of the wafer. Whereas edge-emitting lasers rely on facet mirrors formed at the wafer edge by cleaving or dry etching, the operation of VCSELs is enabled through the use of distributed Bragg reflector (DBR) mirrors for longitudinal optical confinement. VCSELs are advantageous over edge-emitting lasers in that (i) they have a lower-divergence, circularly-shaped laser beam, (ii) may be manufactured using standard fabrication processes such as those used in silicon VLSI technology, (iii) may be tested at the wafer level prior to packaging, and (iv) may be fabricated in dense 2-dimensional arrays for lower cost and higher volume.
As in any laser, the overall structure of a VCSEL is one of two end mirrors on each side of an active region, the active region producing the light responsive to an electric current therethrough. However, the active region is a thin semiconductor structure, and the end mirrors are distributed Bragg reflector mirrors (“DBR mirrors”) comprising alternating layers of differently-indexed material such that light of only the desired wavelengths is reflected. Further general information on VCSELs may be found in the following references, each of which is incorporated by reference herein: Cheng and Dutta, eds., Vertical-Cavity Surface-Emitting Lasers: Technology and Applications, Vol. 10 of Optoelectronic Properties of Semiconductors and Superlattices, Manasreh, ed., Gordon and Breach Science Publishers (2000); Sale, T. E., Vertical Cavity Surface Emitting Lasers, Wiley & Sons (1995); and Dutton, Understanding Optical Communications (Prentice Hall 1998), at pp. 159-161.
As described in Dutton, supra at p. 160, short-wavelength VCSELs having output wavelengths of 850 nm and 980 nm are readily amenable to low-cost fabrication methods, with four separate manufacturers offering low-cost VCSELs by 1998. Commonly, short wavelength VCSELs comprise an active region based on a material system of AlxGa1-xAs/GaAs (for 850 nm) or InxGa1-xAs/GaAs (for 980 nm), as well as DBR mirrors that are also based on alternating layers of AlxGa1-xAs/GaAs. Importantly, such materials for the active region and DBRs are well conformed to each other from a lattice-matching perspective or, if there is a slight mismatch, the resulting strains are desirable ones, as in the case of strained multi-quantum-well (MQW) active layer structures. Thus, for the short-wavelength devices, epitaxial growth of an entire vertical VCSEL structure is readily achieved. Accordingly, short wavelength VCSELs are readily grown in a single-growth process in which, starting with a substrate wafer, successive layers are epitaxially grown (e.g., using molecular beam epitaxy, vapor phase epitaxy, etc.) in a single sweep until the wafer is complete.
Single-growth VCSEL fabrication methods stand in contrast to more complex fabrication methods that are required when, due to nonconformance of DBR material with the active region material system, or due to the presence of complex structures, two or more wafers must be separately fabricated and then fused or bonded together. In addition to the cost and complexity of the multiple wafer growth and bonding process, the results are often less satisfactory than the results of single-growth processes due to the possibilities of mismatches, boundary oxidation, active layer thermal/stress damage, or other singularities along the component wafer boundaries that may lead to reduced device performance and/or reduced device reliability.
Unfortunately, the fabrication of long-wavelength VCSELs (e.g., 1300 nm and 1550 nm) for use in long-distance telecommunications applications has introduced many problems not encountered with short-wavelength VCSELs. One significant problem lies in the lack of availability of suitable DBR materials that conform to known long-wavelength active region material systems. For example, where an InGaAsP/InP material system is used for the active region, it has been found that the use of alternating layers of InGaAsP/InP cannot be readily used for the DBR portion of the VCSEL. This is because the refractive index difference between InGaAsP and InP is relatively small, causing an impractically thick DBR region to be required.
One attempt to solve this problem is discussed in U.S. Pat. No. 6,121,068, which is incorporated by reference herein. In the '068 patent, an InGaAsP/InP material system is used for the active region, whereas alternating layers of SiO2/Si, which are dielectric materials, are used for the DBR regions. These dielectric materials are used instead of alternating layers of InGaAsP/InP, which are semiconductor materials. Because of the more substantial refractive index difference between the SiO2 and the Si layers, a lesser and more practical DBR thickness is realized. However, because the dielectric material has no lattice structure, it may not be epitaxially grown on the substrate material. Instead, a multiple-growth process followed by a wafer bonding process is used. As discussed supra, wafer bonding processes may bring about several disadvantages with respect to device performance and/or reliability.
Another attempt to solve the DBR thickness problem in long wavelength VCSELs is discussed in Babic, “Double-fused 1.52 um vertical-cavity lasers,” Appl. Phys. Lett., vol. 66, p. 1030 (1995), which is incorporated by reference herein, in which alternating layers of AlAs/GaAs are used to form the DBRs. Because the of the more substantial refractive index difference between the AlAs and the GaAs layers, a lesser and more practical DBR thickness is realized. However, because of nonconformities between these materials with respect to the InP substrate material, epitaxial growth of the DBR layers is not practical, and a multiple-growth and wafer bonding process is used to form the VCSEL that may likewise bring about several disadvantages with respect to device performance and/or reliability.
Accordingly, it would be desirable to provide a long-wavelength VCSEL structure that may be fabricated using a single-growth technique. It would be further desirable to provide a VCSEL structure that would be readily adaptable to the use of different materials for the DBR mirrors having differing distances between the DBR layers and other portions of the VCSEL laser cavity.